In recent years, with the spread of computers, inkjet printers have been widely used to print on paper, film, cloth, etc. at offices as well as at home.
Examples of inkjet recording method include a method which allows a piezoelectric element to give pressure that causes a droplet to be ejected, a method which comprises heating the ink to generate bubbles, causing a droplet to be ejected, a method involving the use of ultrasonic wave, and a method which uses electrostatic force to suck and discharge a droplet. As inks for these inkjet recording methods there are used aqueous inks, oil-based inks and solid (melt type) inks. Among these inks, aqueous inks are mainly used from the standpoint of producibility, handleability, odor, safety, etc.
The coloring agents to be incorporated in these inks for inkjet recording are required to exhibit a high solubility in solvents, allow a high density recording and have a good hue and an excellent fastness to light, heat, air, water and chemical, a good fixability to image-receiving materials, difficulty in running, an excellent preservability, no toxicity and a high purity and be available at a low cost. However, it is extremely difficult to seek coloring agents meeting these requirements to a high extent.
Various dyes and pigments have been already proposed for inkjet recording and have been actually used. However, no coloring agents meeting all these requirements have been found yet. Known dyes and pigments provided with color index (C.I.) can difficultly satisfy both the hue and fastness requirements for inks for inkjet recording.
As a dye for enhancing fastness there has been proposed an azo dye derived from aromatic amine and 5-membered heterocyclic amine in JP-A-55-161856. However, such a dye is disadvantageous in that it has an undesirable hue in the yellow and cyan range and thus deteriorates color reproducibility.
The JP-A-61-36362 and JP-A-2-212566 disclose an ink for inkjet recording intended to accomplish both desirable hue and light-fastness. However, the dyes used in these patent references leave something to be desired in solubility in water when applied to water-soluble ink. The dyes disclosed in these patent references are also disadvantageous in that they leave something to be desired in fastness to heat and moisture when applied to water-soluble ink for inkjet recording.
As approaches for solving these problems there have been proposed compounds and ink compositions in JP-A-11-504958. Further, an ink for inkjet recording which comprises a pyrazolylaniline azo dye incorporated therein to improve hue and light-fastness has been disclosed (JP-2003-231850). However, these inks for inkjet recording leave something to be desired in both color reproducibility and fastness of outputted image.
It is known that a pigment ink normally exhibits an excellent durability and water resistance but leaves something to be desired in image gloss or image smoothness on a high density area. A problem solution technique involving the reduction of the particle size of pigment or the coating of the surface of pigment with a resin is known as a related art technique in the art.
It is known, however, that when the pigment particles are finely divided an extent such that the texture gloss of image is sufficient also on a high density area, the durability of the pigment itself deteriorates to the same level as that of dyes.
Unlike photographic process, the image recording method involving inkjet allows free selection of inks of hues other than C, M and Y. To date, SEIKO EPSON CORPORATION has announced and marketed systems comprising a dark yellow ink (dye), a blue ink (pigment) and a red ink (pigment). Further, Canon Inc. announced and marketed a system comprising a red ink (dye).
These inks have been developed to provide an image with a high density and a high quality while suppressing the “ink capacity per unit are”, which is the greatest weak point of inkjet-recording method. However, the greatest difference of inkjet recording, method from image formation using a photographic light-sensitive material is that a black ink can be used as in image recording by printing.
Under these circumstances, the various manufacturers have announced and marketed systems involving the use of ink sets comprising some kinds of inks, including dyes and pigments, having different densities. To date, the printing of letters on document have normally been made by the use of carbon black as a black pigment. However, carbon black is not suitable for the formation of a high quality photographic image. It has thus been desired to use a high quality black dye for the purpose of attaining a desired photographic image quality.
The inventors made studies of inks for inkjet recording comprising dyes (as disclosed in Japanese Patent Application No. 2002-214066, Japanese Patent Application No. 2002-244930, JP-A-2004-83609 and JP-A-2004-83610). In the art, too, the enhancement of fastness of image have been under studies. An inkjet system involving the use of dye ink marketed by SEIKO EPSON CORPORATION in October 2003 has an concept called “Tsuyoink (strong ink)” that is intended to provide a dye ink with an enhanced durability. In particular, various studies have been made of the enhancement of resistance to fading of image by ozone gas occurring in a slight amount in the air, i.e., ozone resistance of dyes. However, it is the status of quo that no water-soluble dye inks which exhibit a stability comparable to pigments when printed on a porous medium to form an image have been obtained yet.